The present invention concerns a flat field lens and more especially a flat field lens capable of producing an image splitting effect.
Flat field lenses are usually constructed in the form of Fresnel echelon lenses. They are used, for example, in or in the vicinity of an intermediate image plane in single lens reflex cameras to brighten the view finder image. In this way, the field lens reproduces the pupil of the camera lens in the pupil of the eye of the observer.
Other optical structural elements in the intermediate image plane serve to make possible a sharp focusing of the view finder image. These consist of either ground glass plates, prism grids, wedge splitters or other image splitting structures.
The field lens and the image splitting structures may be arranged adjacently to each other in the same plane, whereby the central part is used for sharpness of focus adjustment and the periphery for the brightening of the image. Such an arrangement is known, for example, from the Journal Feingeraetetechnik (1963) pages 330-332 and 366-373. The focusing screen described therein comprises in a center annular zone a measuring field which consists of annular prism or cylindrical lenses placed concentrically with respect to each other and arranged on the flat side of a conventional plano-convex lens. These elements produce dual images outside the focusing plate, with the coincidence of the images represent the focusing criterion. The arrangement possesses increased focusing accuracy and enhanced image luminous density in comparison with ground glass focusing arrangements. There is, however, the disadvantage that only a small section of the image is available for focusing. Because of the absence of light scattering elements, observation of the image composition in the usual manner provided by the ground glass observation device is not possible either in the measuring field or in the surrounding Fresnel field.